Flexible magnetic recording media, such as audio and video magnetic recording tapes and computer cartridge tapes, include a magnetic recording layer supported on a nonmagnetic support. The quality of magnetically recorded and reproduced signals depends upon precise, accurate tracking of the magnetic recording media during transport (i.e., during playback, recording, fast forward, and rewind). Irregularities in tracking will generally adversely affect the recording and reproduction of magnetically recorded signals. Additionally, magnetic recording media should be relatively durable so that it is possible to record and play back information repeatedly. In view of these concerns, a backside coating can also be applied to the nonmagnetic support in order to improve the recording, reproduction, conductivity, durability, and tracking characteristics of the media. A backside coating typically includes a nonmagnetic pigment in a polymeric binder along with optional components such as lubricants, abrasives, thermal stabilizers, antioxidants, dispersants, wetting agents, antistatic agents, fungicides, bactericides, surfactants, coating aids, and the like.
One factor affecting tracking characteristics relates to the surface roughness of the backside coating. When the backside coating is too smooth, the coefficient of friction between the magnetic media and the guide surfaces of the recording/playback device increases. As a result, for example, variation in tension can occur at the time of winding, resulting in an irregular transport rate when the magnetic media is subsequently used. Also, with increased friction, air can get trapped under the magnetic recording medium and tape slippage can result. To avoid this and other problems that result from increased friction, the surface of the backside coatings are made with a rough texture to allow air to escape from under the moving tape.
This rough texture can be achieved by the addition of inorganic nonmagnetic particles to the backside coating. Such particles can improve running durability and diminish the lowering of a radio frequency output, for example. Examples of such inorganic particles include silicon dioxide, titanium oxide, zinc oxide, aluminum oxide, chromium oxide, silicon carbide, talc, kaolin, asbestos, mica, boron nitride, calcium carbide, zinc sulfide, and barium sulfate. Inorganic particles, however, are often inadequately bonded to the magnetic media, particularly if they are of an irregular shape and a wide particle size distribution. Furthermore, they can often be so hard that they can be too abrasive and can cause excessive wear to mechanisms in the recording/playback device. Also, if they are too large, they tend to press into the magnetic coating, leaving indentations in the magnetic coating when the medium is subsequently surface treated and/or stored as a roll or wound on a reel. As a result, those electromagnetic properties of the media that depend upon the smoothness of the magnetic coating, e.g., the signal to noise ratio, sensitivity, maximum output level, and the like, tend to deteriorate.
Organic powders have been shown to be useful alone or in combination with inorganic particles in magnetic media backside coatings. These include, for example, a benzoguanamine-type resin, a melamine resin, a phthalocyanine-type pigment, polymethylmethacrylate, carbon black, and graphite. For example, Kokai Patent Application No. SHO 60-121515 (Konishiroku Photo Co.) discloses the use of a powder of a benzoguanamine-type resin. This powder is said to reduce the coefficient of friction without increasing the surface roughness of the backside coating. See also, U.S. Pat. Nos. 5,208,091 (Yanagita et al.) and 4,734,325 (Ryoke et al.), and Kokai Patent Application No. HEI 4-251427 (Hitachi Maxell Co. Ltd.). European Patent Application No. 515,902 A1 (BASF Magnetics Ltd.) discloses a backside coating containing polyolefin spherical particles. These particles contribute to the mechanical stability, e.g., wear resistance and abrasion resistance, of the magnetic media and provide light transparency, which allows for use of the thermo-magnetic duplication process. Such organic particles are generally softer than inorganic particles; however, they can result in higher tape tension and a lack of durability of the backside coating if used as the only "roughness-causing" backside filler. Furthermore, organic particles are often inadequately bonded to the magnetic media.
To avoid undesirable omission of both organic particles and inorganic particles from the backside coating, surface treatments are used to strengthen the binding power between the particles and the binder resin(s). For example, U.S. Pat. No. 4,770,932 (Matsumoto et al.) discloses coating carbon black and titanium monoxide with an isocyanate compound. U.S. Pat. No. 5,219,652 (Shimasaki) discloses coating silicon dioxide particles and particles of a melamine formaldehyde condensation product, for example, with a fatty acid, preferably in combination with a silane or titanate coupling agent. There is still a need, however, for particles, particularly organic particles, that can be used in magnetic media backside coatings for improved performance without significant fallout.